1. Field of the Invention
This invention relates to a resonance-type optical receiver circuit for a optical communication system and to a method of receiving the light waves for the circuit.
2. Description of Related Art
A resonance-type optical receiver circuit includes a tuning circuit interposed between an optical detector and an amplifier. At the tuning frequency of the tuning circuit, the load impedance of the optical detector is high and circuit noise of the optical receiver circuit is suppressed. Accordingly, when a high frequency signal such as a radio signal which is low in ratio (fractional band width) between a required band and a center frequency is transmitted, a high reception sensitivity can be realized by employing a resonance-type optical receiver circuit. Known circuit types of the resonance-type optical receiver circuits include, for example, the parallel tuning type, the transformer tuning type, the mixed tuning type and the third-order band pass tuning type. Those resonance-type optical receiver circuits are disclosed in G. Jacobsen et al., "Tuned Front-End Design for Heterodyne Optical Receivers", Journal of Lightwave Technology, Vol. 7, No. 1, p105, 1989; Kamal E. Alameh et al., "Tuned Optical Receivers for Microwave Subcarrier Multiplexed Lightwave System", Journal of IEEE transactions on Microwave Theory and Techniques, Vol. 38, No. 5, May 1990; Thomas B. Darcie et al., "Resonant P-i-n FET Receivers for Lightwave Subcarrier System", Journal of Lightwave Technology, Vol. 6, No. 4, April 1988; and Noboru Takachio et al., "A Novel Resonance-Type Optical Receiver for High-Speed Optical Heterodyne Transmission System", Journal of Lightwave Technology, Vol. 7, No. 9, September 1989. The resonance-type optical receiver circuits disclosed, however, are publications of the results of research into circuits to prevent noise involved in receiving.
In order to realize a high receiving sensitivity with a resonance-type optical receiver circuit, the load impedance of the optical detector must be high. However, where the load impedance of the optical detector is high, when a light signal of a high level is inputted, the input level to the amplifier in the optical receiver circuit is so high that the output signal of the optical receiver circuit may possibly be distorted. Distortion due to the high level input results a serious problem particularly when a high frequency signal in which a large number of carriers is multiplied optically transmitted.
Further, in a resonance-type optical receiver circuit, it is necessary to have the tuning frequency coincide exactly with the frequency of the received signal. However, under the influence of the quality dispersion of parts, the tuning frequency is sometimes drifts from the frequency of the received signal. This frequency drift matters when a resonance-type optical receiver circuit is actually manufactured or actually used.
Further, in resonance-type optical receiver circuits, an inductor is connected between the optical detector and the ground. Accordingly, when the signal t5 has a very low frequency, the load impedance of the optical detector is low and the level of the signal inputted to the amplifier is very low. Further, with regard to the dc signals, the output of the optical detector is almost equivalent to be short-circuited directly to the ground, so that the dc signal is not inputted to the amplifier. Accordingly, with conventional resonance-type optical receiver circuits, it is difficult to receive a low frequency signal (for example, a base band signal of a television signal or a digital signal, or a modulation signal having a very low carrier frequency) simultaneously with a high frequency signal.